(1) Field of the Invention:
The present invention relates to a novel aqueous dispersion type thermosetting coating composition. More particularly, the present invention relates to an aqueous dispersion type thermosetting coating composition which provides a coating excellent in gloss, smoothness and physical properties without cracking and popping when it is baked and film-formed.
(2) Description of the Prior Art:
Recently, development of paints and coating methods causing no environmental pollution has been desired, and the use of powdery paints and aqueous paints has been expanding steadily. These paints have important characteristics and merits, but they still include various defects and disadvantages as well.
More specifically, the use of powdery paints involves various difficulties. For example, special coating equipment must be used for performing the coating operation when powdery paints are used; hence, the adaptability of powdery paints to the coating operation is much lower than that of conventional solvent type paints. Moreover, coatings formed from these powdery paints are inferior in smoothness and appearance, and it is difficult to form thin coating films having a thickness smaller than 50.mu..
In the field of aqueous paints of the water-soluble type, since it is impossible to form paints having a high solid concentration, it is impossible to form thick coatings by a single coating operation. Furthermore, because of the presence of a number of hydrophilic groups introduced to render the base resin water-soluble, the durability of the resulting coatings is very poor.
Paints of the aqueous dispersion type, called "latex paints" or "emulsion paints" have long been known (see, for example, U.S. Pat. No. 2,904,523, No. 2,994,676, No. 3,002,940, No. 3,025,252, No. 3,170,888, No. 3,224,542, No. 3,657,175, No. 3,687,885, No. 3,839,254 and No. 3,926,874). In these aqueous latex or emulsion paints, since surface active agents are used in large quantities, the resulting coatings are deficient in water resistance, chemical resistance and the like.
As paints overcoming these defects and disadvantages, there have been developed slurry paints (aqueous dispersion type paints) which are formed by suspending a powdery resin in water and are used in the same manner as aqueous paints.
Paints of this type comprise an aqueous dispersion medium and a particulate resin having a size of about 0.5 to about 80.mu., which is dispersed in the aqueous medium at a solid concentration of 20 to 70% by weight.
As the coating method for these slurry paints, there can be mentioned, for instance, a method in which the paint is spray-coated on the surface of an aticle to be coated, water is evaporated and the resin component is cured to form a coating (see U.S. Pat. No. 3,787,230) and a method in which an aqueous dispersion of particles of a nylon resin is spray-coated on an article maintained at a temperature higher than 100.degree. C. to form a coating (see U.S. Pat. No. 2,972,552). As known paints to be used for these coating methods, there can be mentioned, for example, (i) a paint comprising particles of a chlorinated polyester resin having a size smaller than 200 mesh, which are dispersed in water (see U.S. Pat. No. 3,089,783), (ii) a paint comprising particles of a chlorinated polyester polymer dispersed in water containing a surface active agent, an ester of a polyhydric alcohol with a lower aliphatic monocarboxylic acid, an alkaline water softener and a lower aliphatic alcohol (see U.S. Pat. No. 3,446,652), (iii) a slurry paint comprising particles of a resin selected from acrylic resins, epoxy resins, polyester resins, amino resins and polyurethane resins and a water-soluble thickener (see U.S. Pat. No. 3,907,737), (iv) a slurry paint comprising as the binder particles of a powder thermosetting acrylic resin (see U.S. Pat. No. 3,904,795), (v) a slurry paint comprising a reactive flow adjusting agent having a specific structure (see U.S. Pat. No. 4,122,055 ) and (vi) a slurry paint comprising an aqueous medium, resin particles having a size of 0.5 to 80.mu. and aluminum pigment (see British Pat. No. 1,517,834).
As the process for preparing these slurry paints, there are known a process comprising dissolving a resin in a water-soluble solvent, for example, an alcohol such as methanol or ethanol, a ketone such as acetone or diacetone alcohol, an ester such as ethyl lactate or ethylene glycol monomethyl ether acetate or an ether such as ethylene glycol monoethyl ether, and adding the resulting solution to water being agitated at a high speed to precipitate resin particles in water (see U.S. Pat. No. 3,737,401), and a process comprising pulverizing a solid resin according to an ordinary pulverization method and dispersing the pulverized resin in water.
Slurry paints prepared according to these processes are characterized in that they can be applied at a solid concentration as high as about 40 to about 60% by weight, the coating operation can be accomplished by using a conventional coating apparatus for solvent dilution type paints and the only volatile component contained in the paint is the water that is used as the dispersion medium. Furthermore, since this aqueous dispersion medium contains no soluble component, raw materials can be conserved and environmental pollution prevented. Because of these advantages and merits, it is expected that demand for paints of this type will increase remarkably from now on.
However, the slurry paints having the above described merits still have some defects due to their intrinsic properties.
One of the defects is that the use of a water-soluble thickening agent is inevitable for maintaining a stable aqueous dispersion of powder resin particles (usually about 0.05-1.0 wt. % of water-soluble thickening agent is used) in conventional processes. As a result, the thickening agent inhibits melt fluidization of the resin particles in the step of forming a film under heating. Consequently, smoothness and gloss of the resulting coating film are significantly degraded.
Another defect is that atmospheric humidity has a great influence upon the spray-coating since water is used as the dispersion medium. For example, dry-sprayed film is formed if the humidity is low.
Still another defect is that, since the atmospheric temperature in the step of forming a film under heating is 30.degree.-100.degree. C. higher than the boiling point of water, cracking and popping are apt to occur due to rapid evaporation or boiling of water.